The present invention relates to a polarization interferometer, and more particularly to a polarization interferometer which includes a multi-channel photodetector and is suitable for use in an interference spectroscope and a film-thickness measuring apparatus utilizing interference.
In a conventional polarization interferometer, as described in Japanese patent application JP-A-No. 59105,508, an interferogram formed spatially by an optical system in which a Wollaston prism is interposed between a pair of polarizers, is detected by a multi-channel photodetector. In the above interferometer, however, attention is not paid to the non-uniform intensity distribution of a light beam at a light receiving surface and variations in sensitivity at detection elements of the multi-channel photodetector. In more detail, in a conventional polarization interferometer provided with a multi-channel photodetector, the detection elements of the photodetector are placed at different spatial positions, and hence optical paths to the detection elements are different from each other. Owing to the change in optical path length and aberrations of an optical system, the intensity distribution of a light beam at the multi-channel photodetector is not uniform. Moreover, the detection elements do not always have the same photo-sensitivity, and the difference in sensitivity among the detection elements makes the outputs thereof different from one another. Thus, distortion occurs in an interferogram signal delivered from the photodetector.